Method and apparatus for drying a paper web

ABSTRACT

A method and apparatus effect drying of a paper web using a plurality of heated drying cylinders and at least one air impingement module having a hood. The paper web is brought into operative contact with the heated drying cylinders, and at least one of the final moisture content, quality, and cross-direction profile of the paper web is regulated. The regulation is effected by adjusting the efficiency of air impingement drying of the paper web, by adjusting at least one of the blowing velocity of the medium (typically air) blown against the paper web, the humidity of the blowing medium, and the distance of the hood of the impingement module from the paper web. At least one of the final moisture content of the paper web and its quality may also be regulated by adjusting the steam pressure of one or more drying cylinder groups. Typically the final moisture content of the web is measured after the last drying cylinder group and the efficiency of air impingement drying is regulated on the basis of that measurement.

CROSS-REFERENCE TO RELATED APPLICATION

This application is the U.S. national phase of PCT/FI98/00945, filedDec. 4, 1998.

BACKGROUND AND SUMMARY OF THE INVENTION

The object of the present invention is a method and apparatus for dryinga paper web or the like, as defined in the preambles of the independentclaims presented below.

This means that the object of the present invention typically concerns amethod and apparatus for drying a paper web or other similar web in thedryer section of a paper machine or the like, in which dryer section theweb is dried against the heated cylinder surfaces of the dryingcylinders and by means of air impingement drying with at least one airimpingement unit.

Another object of the invention concerns the optimisation of paper webdrying in the dryer section of a paper machine or the like, the dryersection comprising a drying section consisting of one or more dryingcylinder groups and at least one air impingement unit. The invention is,however, intended to be applicable also to other types of dryersections, if necessary.

It is previously known to use twin-wire web transfer and/or single-wireweb transfer in the multicylinder dryers of a paper machine. During thepast 15 years, single-wire web transfer has been used to an increasingextent, in which transfer there is only one dryer wire in each dryingcylinder group, supported on which wire the web passes through theentire group so that the dryer wire presses the web with the help of thedrying cylinders against the heated cylinder surfaces, the web remainingon the side of the outer curve on the turning cylinders or rolls. Thus,in single-wire web transfer, the drying cylinders are outside the wireloop and the turning cylinders or rolls are inside it. Previously knownis the type of dryer sections which consist only of so-called normalsingle-wire web transfer groups in which the drying cylinders are in thetop row and the turning cylinders or rolls in the bottom row.

In order to heat up the drying cylinders, steam is introduced insidethem and the temperature of the drying cylinders is controlled byregulating the steam pressure and/or the rate of flow of the steam. Itis also possible, although rarely implemented, to control the finalmoisture content reached in the dryer section by regulating the speed ofthe machine. In such a case the pressure of the steam is keptconstant—in machines with limited drying capacity usually at maximumpressure—which means that the final moisture content can be increased orreduced by slowing down or speeding up the passage of the web throughthe dryer section.

The most commonly used steam pressure regulating systems are so-calledcascade regulation and thermo-compressor regulation, which aredescribed, for example, in the following publication: TAPPI NOTES,Practical Aspects of Pressing and Drying, Short Course, 1990.

One problem with these conventional-type dryer sections, where drying iscarried out entirely by means of drying cylinders, by using eithersingle-wire web transfer or twin-wire web transfer, has related to theregulation of drying efficiency. In order to achieve the desired finalmoisture content of the web, the drying efficiency of the dryingcylinders is generally regulated by regulating the pressure of the steamsupplied to the cylinders. This type of regulation is relatively slowand does not, therefore, react at optimum speed, for example, to suddenchanges of moisture content in the web originating in the press sectionor wire section. Particularly in connection with a change of papergrade, start-up and web breaks, final regulation of drying to theoptimum level by regulating the pressure of the steam is slow, due tothe considerable mass of the drying cylinders. Methods used inregulation have included dry matter measurement based on IR measurementafter the last drying cylinder, and feedback for controlling steampressures, usually to the main steam cylinder group of the dryersection. This type of regulation has, as such, functioned withoutproblems, where it has been a question of standard production atconstant speed and no web breaks have occurred. Regulation problemsoccur, however, in connection with a change of paper grade, web breaks,or the paper machine start-up phase.

In connection with a change of paper grade, problems are caused inconventional dryer sections by the fact that each drying cylinder has ahigh thermal capacity, due to its great mass, which means that thetemperature of the drying cylinders changes slowly. Because of this, thetemperature changes in the drying cylinders have not been rapid enoughwhere a change of grade is concerned. In some cases, the changesrequired relating to the regulation of drying efficiency have been madeby changing the loads in the press section, but this will also changethe qualities of the paper, which is obviously usually not desirable.Furthermore, when the loads in the press section are changed, thecross-direction profile of the paper web will also change, which means,for example, that there will often be defects in the moisture profile.Due to the foregoing reasons, a web of incorrect final moisture contentor quality standard may be reeled on the reel-up in connection with achange of paper grade. According to solutions known from the prior art,it takes some 15-20 minutes after the change of grade before a balancedstate is again reached. With paper machine speeds of, for example,1500-1800 metres per minute, a large amount of paper, that is, paper ofincorrect quality, is produced during this time. On a wide machine theamount may be 10-20 tonnes.

During a web break, on the other hand, problems arise, for example, dueto the fact that the drying cylinders overheat when no paper that wouldtransfer thermal energy away from the cylinders is supplied to the dryersection. Excessively hot cylinders cause problems in tail threadingafter a web break, as the tail threading cord adheres to the hotcylinders. In addition, excessively hot cylinders overdry the tailthreading cord, which makes the cord brittle and causes it to lose itsstrength properties, which may cause problems in tail threading.Furthermore, at the stage when the tail threading cord broadens after aweb break, it takes a long time before the drying cylinders return to anequilibrium temperature due to the drying cylinders' slow ability tochange their temperature, that is, their high thermal capacity. In caseof a web break, there was previously no other way of bringing thesituation under control than by reducing steam pressures for theduration of the web break. The result of this has in turn been that thefinal moisture content after a web break has not conformed to thedesired values. Furthermore, it has taken a long time before thesituation could be returned to correspond to normal running conditions.

At the paper machine start-up stage, the steam pressures suitable for aparticular paper grade are usually first taken from a memory in whichadjustment values that have proved good in earlier, correspondingrunning situations have been collected, for example, in tables, and thesteam pressures in the drying cylinders are controlled by means of theabove data. The steam pressures selected and their time staggering, orchange, may also be based on computational models and values obtainedthereby. Conventionally, when the web is first taken to the machine, thesteam pressures used are slightly below the optimal pressures, and afterthis the steam pressures are increased to the desired level. The highthermal capacity due to the great mass of the drying cylinders makesstart-up slow, so that it takes a long time before the desired situationis reached. This is problematic because during the start-up stage alarge amount of paper of an incorrect type is produced.

The problems described above are thus mainly due to the fact that thethermal capacity of the drying cylinders is high and there is a longtime-lag before they achieve the temperature changes required.

The dryer section is the part of the paper machine that uses the mostenergy. It can be said that as much as over two thirds of the energyconsumption of a paper machine takes place in the dryer section. Thedryer section should, therefore, be used as economically as possible,that is, in such a way as to achieve as high an evaporation efficiencyas possible, and a high-quality drying result and low energyconsumption. Drying must be uniform also in the cross direction of theweb. Cylinder drying is currently the most common drying method used.

The web cannot be profiled, that is, evaporation cannot be regulated soas to be uniform in the cross direction of the web by means ofregulating the steam pressure of the cylinders or the speed of themachine. In the dryer section, or even before it, streaks often form atsome points on the web parallel to the travel direction of the web, inwhich the web dries more than at other points. Variations in themoisture profile of the web to be dried when it arrives at the airimpingement unit are not only dependent on non-uniform drying in theactual dryer section, but are often due to non-uniform dewatering in thepress section. Variations in the moisture profile may also be due to anon-uniform solid matter profile appearing already on the wire section.Changes in the need for drying also arise in connection with a change ofgrade. Moisture profile defects such as these must be rectified.

Previously, attempts have been made to rectify moisture profile defectsby spraying water jets directed from jet pipes or nozzles fitted acrossthe web at the said areas in order to render the web moisture leveluniform. Adding water to the web is obviously less advantageous from thepoint of view of energy consumption, because the water sprayed for thepurpose of regulation must be evaporated again from the web at a laterstage. An alternative way of rectifying a moisture profile defect hasbeen to use infrared dryers fitted across the web, which will evaporatewater especially from the areas of the web with the highest moisturecontent. Infrared dryers consume relatively large amounts of energy.

In cylinder drying, the web to be dried is passed over the surface ofthe drying cylinder, pressed by the wire, which means that the sidewhich is against the web cylinder at any time heats up and dries moreefficiently than the other side of the web. In modern, fast machines, inwhich the web is dried using single-wire web transfer, only one and thesame side of the web comes into contact with the cylinder surface ineach dryer group, and thus dries more efficiently than the other side ofthe web. One-sided drying of the paper web is even further emphasised ifthe paper web is passed to the drying cylinders with the same sidealways in contact with them, also in the different drying cylindergroups. Paper dried in this one-sided manner tends to curl when in theform of sheets, which causes major problems in the finishing of thepaper.

To solve the foregoing problem of one-sidedness, that is, curling, ithas been suggested that a so-called reversed group be fitted in thedryer section, through which the web is passed supported on the wire sothat the other, less efficiently dried side, will run in contact withthe drying cylinders. This solution requires a different type of dryersection construction in which, deviating from other dryer groups, thedrying cylinder and wire loop are fitted to run below the web. In thiscase, the paper broke formed during a web break or start-up falls downinto the pockets formed by the wire loop, between the drying cylindersand the wire, from where it may be difficult to remove the broke. Due tothe difficulties relating to the removal of broke, the runnability ofthis type of a reversed dryer section is poor in connection with webbreaks and start-ups. In conventional single-wire web transfer, on theother hand, the wire loop is fitted to run above the web, in which casethe broke formed in connection with a web break falls freely below themachine, from where it can easily be removed.

The aim of the present invention is in fact to achieve an improvement tothe problems described above.

It is a particular aim of the invention to create a method and dryersection, in which rapid regulation of drying efficiency is possible, forexample, in connection with a change of paper grade, web breaks andstart-up situations.

Another aim is to achieve an energy-efficient method and apparatus whichmake possible rapid adjustment of overall evaporation, for example, inconnection with a change of paper grade, a web break and start-up.

A further aim is to achieve a method and apparatus which make possibleprecisely targeted adjustment of evaporation, such as adjustment ofprofiling or evaporation also in the cross direction of the web.

Yet another aim is to achieve a method and apparatus, by means of whichone-sided drying of the web and the consequent curling can be minimised.

Yet another aim is to achieve the type of method and apparatus forregulating the drying of a paper web which allows easy maintenance ofthe dryer section, rapid removal of broke and thus good runnability.

In order to achieve the above-mentioned aims, the method and apparatusrelating to the invention are characterised by what is specified in thecharacterising parts of the independent claims presented below.

Instead of a dryer section based on conventional drying cylinder drying,the invention relates to a dryer section which applies both airimpingement drying and drying by means of drying cylinders. As regardsthis type of dryer section, reference is made, for example, to theapplicant's Finnish patent applications FI 971713 and FI 971714.

In this application, air impingement drying refers both to airimpingement drying, directed directly to the web, and to through-flowdrying, being effected through the wire or a corresponding conveyorfabric. So-called through-flow drying, which is particularly well-suitedfor drying porous webs, is also included within the scope of the airimpingement drying referred to in the invention. According to theinvention, air impingement drying can be directed at a web passing overa large-diameter cylinder, roll, suction roll, through-flow cylinder orother curved surface. Air impingement drying can also be directedlinearly, for example, to a web supported by a wire or belt which runssupported on rolls or blow boxes. The linearly running web may bearranged to run in a horizontal, vertical or inclined position. Hot airor superheated steam are preferably used as the medium in airimpingement.

In order to achieve the aims presented in the foregoing and appearinglater, a typical method relating to the invention for drying a paper webor the like in a dryer section comprising at least one air impingementunit, is characterised mainly in that, according to this method, thefinal moisture content of the paper web and/or any othermachine-direction quality and/or the cross-direction profile areadjusted by regulating the efficiency of air impingement.

The apparatus relating to the invention for drying a paper web or thelike in a dryer section comprising at least one air impingement unit ismainly characterised in that it incorporates a measuring instrument formeasuring the final moisture content of the paper web and/or othermachine-direction quality and/or the cross-direction profile, and meansfor regulating the blowing force of at least one air impingement unit onthe basis of the measurement result.

According to the invention, air impingement drying can be used duringdifferent transient stages, such as those concerning a change of grade,a web break and start-up, for controlling changes in drying capacity,and for eliminating or at least minimising the problems occurring duringthese stages.

The advantages achieved by means of the invention are based especiallyon the fact that air impingement reacts extremely rapidly to adjustmentmeasures, which means that it can be used for the rapid adjustment ofdrying efficiency required by, for example, a change of grade, a webbreak or start-up. In addition to the rapid adjustment of dryingefficiency, the steam pressures of the drying cylinders in the dryersection are advantageously also regulated at the same time.

In the regulation of the drying efficiency of the dryer section anddrying method relating to the invention, an optimisation algorithm knownas such, may be used, which algorithm optimises the drying costs and/orqualities of the paper. According to the invention, for example, MPC(Model Predictive Control), that is, model predictive multivariablecontrol can also be used.

When air impingement is used in accordance with the invention forregulating drying efficiency, one or more of the various air impingementparameters can optionally be adjusted as desired in the air impingement:for example, the blowing rate, the temperature of the blowing medium,the humidity of the blowing medium, and the distance of the air hoodfrom the web (advantageous especially during web breaks). The airimpingement hoods can also be built of machine-direction segments, inwhich case it will be possible to adjust and/or, where necessary, toclose each segment separately. In order to adjust the cross-directionprofile, the air impingement hoods can also be divided intocross-machine direction segments in which the above-mentioned airimpingement parameters can be adjusted either together or separately.

In connection with a change of grade, the set values required by the newpaper grade are usually known in advance, that is, for example, thesteam pressures of the drying cylinders by means of which the desiredend product is achieved. The steam pressure may thus be set at thedesired level immediately at the start of the change of grade orgradually, also when applying the method relating to the invention.However, since this type of adjustment is slow, air impingement isregulated at the same time according to the invention, either on thebasis of an existing air impingement model, by means of which therequired regulation measures are calculated, or through continuousfeedback regulation. As the drying efficiency of the cylinders graduallychanges, the change is compensated by an opposite change in the dryingefficiency of the air impingement.

At the stage of starting up a paper machine, the drying cylinders arefirst heated in accordance with known heating sequences. The airimpingement hoods are preheated in a corresponding manner. After this,the running parameters may be set as desired in accordance with thepredetermined values or the drying simulation calculation. By means offeedback, the values of the drying cylinders and air impingement canthen be controlled so that the desired quality parameters are obtained.

When the running of a completely new paper grade is begun, the setvalues that have proved good for the closest paper grade among previousrunning situations are first selected from the memory, and after this,by utilising these set values and by applying feedback, air impingementand advantageously also steam pressures are regulated so that thedesired values are obtained for the new paper grade.

In case of a web break, the steam pressures of the drying cylinders aredropped, the air impingement hoods are opened and the broke is directedfor removal by means of the broke conveyors. The hoods' own controlsystem takes care of by-pass air circulation inside the hood, gas feedregulation, and regulation of the blowing rate, exhaust air and freshair. When the track is restarted after tail threading, these measuresare obviously carried out in reverse order and the required regulationof drying efficiency is carried out by means of air impingement.

The invention is naturally also applied in regulating drying efficiencyduring normal running, in addition to the special stages describedabove. According to the invention, the quality of the paper can beoptimised continuously, also in the quality sense, while at the sametime applying the cost data. This means that the position of the airimpingement unit relating to the invention in the dryer section may alsobecome a regulating parameter.

The position of air impingement has been taken into account in oneparticular embodiment of the invention, in which the dry matter contentof a paper web coming from the press section of the paper machine andguided through at least one drying cylinder group, and dried to a drymatter content of preferably, for example, over 70%, even over 75%, isregulated by passing the paper web, supported on the wire or the like,through a slot-like space formed between

a curved or linear air impingement hood extending across one or morewebs, and

a curved or linear surface, such as a cylinder, roll or vacuum boxextending across one or more webs, and

by blowing towards the paper web, from the said one or more airimpingement hoods in the said slot-like space, several successive hotair jets of predetermined drying efficiency and in cross direction tothe web. If so desired, steam jets can be used instead of air jets, inwhich case, however, consideration must be given to the special demandsmade by hot steam on the hood construction of the hood as well as, forexample, its sealing requirements.

The air impingement hood refers to any box-like construction known assuch, from which hot air or steam jets are blown through holes, slots orother nozzles onto the web.

The vacuum box refers advantageously to a box-like construction creatinga low vacuum of approximately 100-400 Pa, preferably 200-300 Pa, betweenthe vacuum box and the wire/web, the side of the construction on theweb's side being mainly planar. The purpose of this relatively lowvacuum is to prevent the detrimental detachment of the web from thewire. The aim is, by means of the vacuum, to prevent, the web fromflapping, for example, due to the blowing from above, and thus cominginto contact with the air impingement hood. The aim is to guide the webin a controlled manner through the slot formed between the boxes. Thelow vacuum required can preferably be created by means of a blow box,such as the one described in FIG. 4a below, or by means of a suctionbox.

A typical paper machine dryer section can be divided into three parts:

a first part, in which the paper web is primarily heated, however, atthe same time increasing the dry matter content of the web typicallywithin the range of 40-60%,

a second part, in which most of the free water in the web is removedthrough uniform evaporation in the drying cylinder groups through whichthe web is passed as single-wire or twin-wire web transfer, and in whichthe dry matter content of the web increases typically within the rangeof 45-85%, and

a third part, in which the web is finally dried by means of the dryingcylinders, and in which the dry matter content of the web typicallyincreases to a range of 75-98%.

In a special embodiment of the invention, the air impingement unit whichenables the precisely targeted adjustment of drying can be fitted in theoptimum area in the dryer section with a view to the regulation ofdrying and energy consumption, that is, for example, in an area wherethe web has already reached a dry matter content of >70%, preferably75%. The said area is located at the end of the dryer section, typicallybefore the last or second last drying cylinder group, a typical dryingcylinder group in a dryer section provided with single-wire web transfercomprising approximately 3-8 drying cylinders.

The relative drying efficiency of the drying cylinders falls once theweb has reached a dry matter content exceeding 70%, typically 75%, thatis, when the major part of the readily evaporated water has been removedfrom the web. By means of air impingement, water bound more closely inthe web can be removed, even from a web as dry as this, with good dryingefficiency.

In the solution relating to the special embodiment of the invention, thenew observation is utilised that by means of air impingement, as anefficient drying impulse,

it is possible to influence web drying advantageously, not only at theinitial stages of drying, that is, while raising the temperature of theweb, but expressly also at the final stages of drying. The use of thedrying cylinders is most effective, from the energy efficiencyperspective, more or less at the middle of the dryer section;

it is possible to influence the profiling of the web when the dry mattercontent of the web is more than 70%, even more than 90%, until the finaldry matter content is reached. With previously known methods, attemptshave been made to influence the profiling of the paper web at the end ofthe dryer section.

it is possible to eliminate the curling of paper in a preferred mannerat the final stage in the dryer section, where the dry matter content ofthe web is already over 70%, typically over 75%, in an area after whichthe paper web no longer tends to dry one-sidedly as readily as in themiddle of the dryer section. The use of reversed groups at the end ofthe dryer section has previously been suggested for the control ofcurling.

It has now been found that the optimum area for the combined raising ofdrying efficiency, profiling and curling control by means of airimpingement is located at the end part of the dryer section, where thedry matter content of the paper web is more than 70%, preferably morethan 75%, up to a dry matter content of about 95%, preferably within therange of 75%-85%. Even a very short, effective drying impulse lastingless than 1 second, or even less than 0.5 second, is often sufficientfor regulating drying. A short, efficient drying impulse can be achievedby linear air impingement, over a length of 1-20 m, preferably 5-10 m.

Generally, it may be advantageous to carry out final drying after airimpingement by means of drying cylinders, with one or two dryingcylinder groups. In certain special cases, the air impingement unitrelating to the invention can also be fitted at the very end of thedryer section. This should be done especially when the final dry mattercontent of the web being dried remains at 90% or only slightly abovethis.

The regulation of drying efficiency is usually based on the dry mattercontent of the web measured after the dryer section, irrespective of thecause of the need for a change in drying. Measurement may obviously alsotake place elsewhere, before or after the air impingement unit. Thedrying efficiency of the air impingement unit relating to the inventionis also adjusted on the basis of the dry matter content measured. Thedrying efficiency of a typical air impingement unit relating to theinvention is regulated by adjusting the temperature, moisture content orvelocity of the air jets blown.

The blast air used in the air impingement unit is preferably blast airconducted from the paper machine room, or in a dryer section closed by ahood, the return air of the hood, or the air impingement device's ownreturn air. The temperature of the blast air is raised and/or itshumidity level is lowered before blowing towards the paper web. Thereturn air of the various air impingement units can be heated by meansof a common burner, such as a gas or oil burner or other similar heaterfitted in a separate space adjacent to the dryer section. On the otherhand, an individual burner or the like may be integrated into each airimpingement unit or part of a unit, in the web direction or in the crossdirection of the web, which means that the different air impingementunits or their parts can be adjusted independently of each other.

The air impingement hood or unit relating to the invention preferablyblows hot air, the temperature of which is set between 40° C.-500° C.,advantageously 200° C.-400° C., depending on the drying efficiencyrequired at any time. The moisture content of the air jets typicallyvaries between 0-300 g H₂O/kg of dry air.

The drying efficiency of the air impingement unit can, however, also beregulated by adjusting the velocity of the air jets. In such a case thevelocities of the air jets are typically maintained between 40-200 m/s,preferably between 50-150 m/s, and most preferably between 70-120 m/s.

By means of the efficient hot air or steam jet of the air impingementsystem relating to the invention, it is possible to regulate dryingdirected at the paper web extremely rapidly, practically without delay.The change of adjustment taking place in the air impingement hood isseen to its full extent in the paper web within a few seconds.

The temperature of the hot air can be regulated simply by adjusting thefuel valve of the burner. No time is taken for raising or lowering thetemperature of the device itself, as in drying with a drying cylinder.By means of air impingement according to the invention, dryingefficiency can be changed by 20-100% extremely rapidly. A completechange can typically be achieved in less than 30 seconds, usually inless than 10 seconds, which is only a fraction of the time required forbringing about the same change with conventional cylinder drying.Regulation by cylinder drying takes several minutes.

In the dryer section, two air impingement units can also be fitted insuccession within the optimum range for regulating air impingement, inwhich case their drying efficiency can be adjusted either separately ortogether in order to achieve an optimal drying result. It is usuallyadvantageous to fit successive air impingement hoods in the dryersection in such a way that the drying efficiency of the air jets comingfrom the first air impingement hood is on average greater than thedrying efficiency of the air jets coming from the air impingement hoodfollowing it.

Each air impingement unit relating to the invention preferably comprisesseveral adjacent rows of nozzles which are formed by blow nozzles fittedin succession across the web. These nozzles can be arranged so as to beadjustable all by the same adjustment, each nozzle separately, or aspecified group of nozzles separately. It is often advantageous todivide the air impingement unit relating to the invention into severalsegments in the cross direction of the web, in which case the nozzles inthe different segments can be adjusted separately. The segments may beas narrow as 100 mm. Typically, the width of the segments varies between500 mm-2000 mm.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in greater detail in the following, withreference to the appended drawings, to the details of which theinvention is not intended to be strictly limited in any way, and inwhich

FIGS. 1a/1 b show a diagrammatic view and an example of a dryer sectionin a paper machine in which the method relating to the invention is usedfor adjusting the drying cylinders and regulating air impingement;

FIG. 2 shows a table which shows, as an example, the effect of theblowing temperature and blowing velocity of air impingement on dryingcapacity;

FIGS. 3a-3 d show diagrammatically previously known air impingementunits fitted in conjunction with drying cylinders or suction rolls;

FIG. 4 shows diagrammatically a cross-section in the web direction of alinear air impingement unit applying the invention when fitted betweentwo drying cylinder groups;

FIG. 4a shows diagrammatically an enlarged view of the vacuum box shownin FIG. 4;

FIG. 5a shows diagrammatically a part of the nozzle surface of the airimpingement hood shown in FIG. 4;

FIG. 5b shows diagrammatically a vertical cross-section of a part of theair impingement hood shown in FIG. 4;

FIG. 6 shows, in the same manner as FIG. 4, a second linear airimpingement unit relating to the invention;

FIG. 7 shows, in the same manner as FIG. 4, a third linear airimpingement unit relating to the invention;

FIG. 8 shows a typical drying efficiency curve of the drying cylindersin a dryer section provided with single-wire-wire web transfer, and

FIG. 9 shows the moisture profile curve of the paper web before andafter profiling with the apparatus relating to the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

In FIG. 1a, the invention is described in conjunction with the firstdryer section, that is, the dryer section fitted immediately after thepress section. This is not, however, intended in any way to limit theinvention to concern only the first dryer section. The invention isfully applicable also to the intermediate dryer section or last dryersection. In the present specification and claims, the term dryer sectionin fact refers to the dryer section as a whole and its parts, such asthe first, intermediate and last dryer sections, unless otherwisespecified.

FIG. 1a shows a particularly advantageous dryer section solution inwhich the invention is applied. In FIG. 1a the paper web W is conveyedfrom the press section (not shown) of the paper machine to the beginningof the dryer section, to its first dryer unit R₁ by means of the pressfelt. In the case of FIG. 1a the first dryer unit is a planar dryerunit, that is, the linear air impingement unit M₀, which comprises anair impingement hood 10, from which hot air and/or steam is blown ontothe web W running on the wire 12. The web runs below the hood 10 on thehorizontal run of the wire 12, which is supported on support means 14.The said means supporting the horizontal run of the wire 12 and thusalso the web, consist, for example, of grooved rolls and/or suction orblow boxes. By means of the air impingement module M₀ of the dryer unitR₁, an intensive drying energy impulse is directed at the web W from theair hood.

In the air impingement module M₀, the paper web runs linearly on thehorizontal plane, supported by the upper run of the dryer wire 12, insuch a way that it is not subjected to any great changes of directionand thus no great dynamic forces are exerted on it, which might cause aweb break in the web, which is still relatively moist and thus fragile.Inside the air hood 10 is a nozzle arrangement for the purpose ofbringing about air impingement, by means of which arrangement hot dryinggases such as air or steam are blown onto the upper surface of the web.In the dryer unit R₁ it is possible, additionally or alternatively toemploy radiators such as infrared heaters. The air impingement devicesand/or radiators of the dryer unit R₁ can be arranged so as to beadjustable as to their efficiency in the cross direction of the web, forthe purpose of achieving cross-web profiling in the web W. Although theair impingement module M₀ shown in FIG. 1a is a horizontal model, it isobvious that in its place, alternatively or in addition, other types ofunits may also be used, such as a drying device based on air impingementtaking place above the suction roll. At this point, the air impingementmodule M₀ can even be replaced by a cylinder group, if air impingementdrying can be utilised in accordance with the invention at a later stagein the dryer section. In the planar dryer unit M₀, an impermeable beltcan be used instead of a conventional wire 12.

In FIG. 1a the first so-called normal (non-reversed) dryer unit R₂,comprising a single-wire-wire 16 drying cylinder group, is fitted afterthe planar dryer unit M₀. The wire 16, like most of the other wires, isshown only partly in the figure. The second dryer unit R₂, as well asthe next similar, so-called normal, downwards-open dryer units R₄, R₆,R₈, R₉ and R₁₀ comprised of a single-wire cylinder group, incorporatethree or four contact drying cylinders 20 fitted in the top row andheated by means of steam, and three or four turning suction rolls 22,for example a VAC-roll, fitted on the bottom row. The paper web W to bedried comes into direct contact with the surfaces of the dryingcylinders 20 heated with steam. On the turning suction rolls 22, the webW remains on the side of the outer curve of the dryer wire 16.

In the dryer section shown in FIG. 1a, the dryer unit R₂ is followed byan air impingement dryer unit R₃, which comprises two contact dryingcylinders 24, 24′, and an air impingement module M₁, which in turncomprises a large-diametral D₁ air impingement/through-flow cylinder 26with bores in its envelope, subsequently called a large-diametercylinder, and openable hoods 28, 28′ partly covering the envelope of thesaid large-diameter cylinder 26. A dryer wire 30 is fitted to passaround the contact drying cylinders 24, 24′ and the large-diametercylinder 26.

The air impingement module M₁ of the drying cylinder R₃ is fitted in thebasement premises BP below the floor level K₁ of the paper machine room,and mounted on the floor level of the said space. The central shafts ofthe contact drying cylinders 24, 24′ of the air impingement dryer unitR₃ and the similar following air impingement dryer units R₅ and R₇relating to the invention are preferably located substantially at thefloor level K₁ of the paper machine room, or close to it, mostpreferably slightly above it.

The paper web W to be dried is passed from the dryer unit R₂ comprisedof the first drying cylinder group, preferably as a closed transfer, tothe first contact drying cylinder 24 of the next air impingement dryerunit R₃, after which the web W is passed on the wire 30 of the unit R₃over the large-diameter cylinder 26 of the air impingement module M₁, ina considerably wide sector b≈180°-280°, supported by the dryer wire 30,and from there on to the second contact drying cylinder 24′ of the saidunit R₃. From this second cylinder 24′, the web W is transferred, againpreferably by a closed transfer, to the next so-called normal dryer unitR₄ comprised of a drying cylinder group, which unit is basically similarto the dryer unit R₂ described above. After this follows another airimpingement dryer unit R₅ equipped with a large-diameter cylinder, thesaid unit being similar to the air impingement dryer unit R₃ describedabove, and the large-diameter cylinder 26 of which is also located inthe basement premises BP. After the dryer unit R₅, the web W isconveyed, still preferably by a closed transfer, to the next dryer unitR₆ comprised of a drying cylinder group, the said unit being similar tothe dryer units R₂ and R₄. After the dryer unit R₆ follows the third airimpingement dryer unit R₇ equipped with a large-diameter cylinder, thelarge-diameter cylinder 26 being also located in the basement premisesBP. After the air impingement dryer unit R₇ follow three successiveso-called normal dryer units R₈, R₉ and R₁₀, comprised of dryingcylinder groups, each incorporating three or four drying cylinders 20.From the last dryer unit R₁₀ the web W_(out) is fed out of the dryersection to the reel-up or finishing unit (not shown).

Both the so-called normal dryer units R₂, R₄, R₆, R₈, R₉ and R₁₀, andthe air impingement dryer units R₃, R₅ and R₇ open downwards, whichmeans that paper broke is easily removed from them, on the brokeconveyor below the units, or to the pulper below. Beneath the airimpingement modules M₀, M₁, M₂ and M₃, in the basement premises BP, oreven beneath these premises, there is ample space for various kinds ofequipment such as ducts, through which the heating medium, such asheated air or steam is introduced, for example, into the hoods 28, 28′of the modules M₁, M₂ and M₃. Above the dryer units R₂-R₁₀ there is aventilated dryer section hood 32, known as such.

The method of regulation relating to the invention is described in thefollowing with reference to FIGS. 1a and 1 b. FIG. 1b shows a simplifiedrepresentation of the control circuits of FIG. 1a. FIG. 1b omits thefirst dryer unit R₁ of the dryer section, that is, the planar airimpingement module M₀, but it can be employed in the same manner as theair impingement modules M₁, M₂, M₃ for the rapid regulation of dryingefficiency required by the dryer section in a manner described in thefollowing. In FIG. 1b, the same reference numbers are used forcorresponding parts as in FIG. 1a.

A measuring device 34 is located after the last dryer unit R₁₀ of thedryer section, which measuring device measures the moisture content ofthe paper web W. If so desired, regulation may obviously also be basedon the measurement of other quality parameters of the paper web, such asweb brightness, or on cross-direction profile measurements of the paperweb W. From the measuring device 34, the measurement results are fedthrough the control unit 36 to the steam pressure control 38 of thedrying cylinders 20, 24, 24′, and to the air impingement control unit40, by means of which the drying of the paper web W is controlled insuch a way that the rapid action required by drying in connection, forexample, with a web break, a change of grade and the start-up stage arecarried out by means of the air impingement modules M₀, M₁, M₂, M₃ ofthe air impingement units, while the steam pressure control unit 38ensures that the steam pressures of the drying cylinders 20, 24, 24′ areadjusted to the desired level.

According to the invention, the air impingement control unit 40 can beused to control the air impingement parameters of the air impingementmodules M₀, M₁, M₂, M₃, such as the temperature, velocity or humidity ofthe blast air/steam, or the distance of the hood from the web, either inone of the modules M₀, M₁, M₂, M₃ at a time, or in several modulessimultaneously. In the case shown in FIGS. 1a and 1 b, each of the airimpingement modules M₀, M₁, M₂, M₃ has its own control unit 40 ₁, 40 ₁,40 ₂, 40 ₃ for transmitting their control parameters. When rapid actionis required for regulating drying efficiency, either one or several ofthe air impingement modules M₀, M₁, M₂, M₃ are controlled so that thedesired moisture content and other properties of the paper are obtainedrapidly. In the air impingement modules M₀, M₁, M₂, M₃, one or more ofthe following controlled variables can be controlled at the same time toachieve the desired regulatory action: blowing velocity, temperature ofthe blast air, humidity of the blast air and/or distance of the air hoodfrom the web W. Any air hood or its segment may, on the other hand, beswitched off completely if desired.

On the other hand, by means of the steam pressure control 38 of thedrying cylinders it is possible to control the steam pressures of theconventional drying cylinders 20 or the contact drying cylinders 24, 24′of each dryer unit R₂, R₃, R₄, R₅, R₆, R₇, R₈, R₉, either by controllingthe steam pressures of one or more drying cylinders or contact dryingcylinders separately, or by controlling the steam pressures of one ormore cylinder groups separately by means of regulating elements 38 ₂, 38₂′, 38 ₂″, 38 ₃, 38 ₄, 38 ₅, 38 ₆, 38 ₇, 38 ₈, 38 ₉, 38 ₁₀, 38 ₁₀′, 38₁₀″, on the basis of the control signals transmitted. Control of thesteam pressures of several dryer groups simultaneously also falls withinthe scope of the invention, because often, for example for reasons ofcost, there are fewer steam groups than wire groups.

During normal operation, drying and/or any other quality parameter canbe kept within the desired limits by adjusting, for example, only one ortwo of the air impingement modules M₀, M₁, M₂, M₃. One or more of theblasting parameters of these modules or the distance of the air hoodfrom the web W are controlled on the basis of the measurement resultsobtained from final moisture content measurement 34 in such a way thatthe desired final moisture content or quality properties are obtained.If necessary, the final moisture content or other quality parameter mayalso be corrected by regulating the steam pressure of one or morecylinders 20, 24, 24′ or cylinder groups of the drying cylinder unitsR₂. . . R₁₀, through use of steam pressure control 38.

The blasting parameters are controlled in the air impingement controlunits 40, 40 ₀, 40 ₁, 40 ₂, 40 ₃, either on the basis of an optimisationalgorithm or by utilising multivariable control. If necessary, both airimpingement and the steam pressures of the drying cylinders can becontrolled simultaneously in order to achieve the desired qualityparameters.

When the paper grade is changed, the information on the desired grade istaken to the control system 36 from the memory 42, for example from atable, and on the basis of this information on the grade, the steampressures of the cylinders 20, 24, 24′ of the selected drying cylindergroups R₂. . . R₁₀ are controlled by the control unit 38, by means ofthe tabular values and a calculation formula, for example in such a waythat the steam pressures are regulated gradually to the desired level.The rapid regulation required by the change of grade is effected at thesame time, in order to obtain paper possessing the desired qualities asrapidly as possible, by means of air impingement control 40, either bycontrolling air impingement on the basis of a target value bycalculating the required blasting parameters, or by continuous feedbackcontrol from measurement point 34.

At the start-up stage, the drying cylinders 20, 24, 24′ are first heatedaccording to known heating sequence parameters, and at the same time thepreheating of the hoods 10, 28, 28′ of the air impingement modules M₀,M₁, M₂, M₃ is carried out. After this, the running parameters are set onthe basis of the predetermined values, and air impingement is controlledby means of the feedback control feature, in order to obtain the desiredquality rapidly, after which air impingement continues to be regulatedwhile the steam pressures of the cylinders change, until they havereached the target values.

During a web break, the necessary adjustments are carried out, that is,the steam pressures of the drying cylinders 20, 24, 24′ are adjusted tothe break steam settings applied during a web break, and air impingementmodules M₀, M₁, M₂, M₃ are switched on at the same time on the basis ofmachine automation control. The internal control systems 40 ₀. . . 40 ₃take care of by-pass circulation inside the hood, reducing gas feed, andreducing blowing velocity and closing off exhaust air and opening upfresh air feed. After the break, the above-mentioned measures arecarried out in reverse order, and rapid quality correction is carriedout by adjusting the air impingement parameters, until the dryingcylinders 20, 24, 24′ reach the desired target values.

When a new paper grade is to be run, the parameters of the paper grademost similar to the new grade are selected from the memory 42, forexample from a table therein, and the air impingement modules M₀, M₁,M₂, M₃ and the steam pressures of the drying cylinders 20, 24, 24′ areadjusted on the basis of these. By means of feedback control, thequality parameters are then adjusted so as to obtain paper possessingthe desired properties.

Also when regulating the moisture content, tension or other qualityprofiles of a paper or board web in different operational situations, itis advantageous to do so in accordance with the invention. It isworthwhile to carry out at least rapid corrections of thecross-direction profile by regulating the efficiency of air impingementin the cross direction. The corrections made can then in the long termbe carried out by means of other profiling devices, should this, forexample, be more cost-efficient.

Table 1 shows the effect of the blowing temperature and blowing velocityof the air impingement module M₁, M₂, M₃ on drying capacity. The tableis based on the results obtained by means of a dryer section simulationprogramme verified by measurement results, in a case where the airimpingement of one or two modules M₁, M₂, M₃ relating to FIG. 1a isbeing regulated. In the table, the air impingement modules are markedwith reference markings AI1 and AI2. In the simulation programme, thespeed of the paper machine was assumed to be 2000 m/min and the width ofthe paper web 9.5 m and the grammage of the paper web 41.4 g/m². Thefirst part of the table shows, as marked above the table, the effect ofregulation of blast air temperature on drying, when the air impingementvelocity is 90 m/s. The second part of the table shows, in acorresponding manner, the effect of regulation of air impingementvelocity on drying, when the blast air temperature is 350° C. In thetable it can be observed, for example, that by means of one airimpingement unit, at a blowing velocity level of 90 m/s, a moisturecontent correction of the magnitude of about 2 percentage points can beachieved by varying the temperature of the blast air between 250°C.-350° C. Similarly, by increasing the blowing velocity from 60 m/s to120 m/s, a change of about 2.5 percentage points is obtained with one,and of 5 percentage points with two air impingement units.

In applying the invention, the drying efficiency can thus be increasedand the cross-direction profile adjusted by installing air impingementunits blowing hot air or hot/superheated steam in the dryer section, inconjunction with the cylinders, rolls or planar wire runs therein. Inthe following, FIGS. 3a-3 d show some air impingement modules differingfrom the preferred air impingement modules M₁-M₃ for applying theinvention shown in FIG. 1a.

FIG. 3a shows an air impingement concept in which, in a reversed dryersection, a curved air impingement hood H, conforming to the surfacecontour of the suction roll 22 preceding the drying cylinder 20, isfitted over the suction roll. The hot air jets of the air impingementhood, which are not shown in greater detail, are directed onto that sideof the paper web which is against the hot cylinder surface on a dryingcylinder. In the case of FIG. 3a, the air impingement hood is fittedoutside the wire loop, which thus allows for easy maintenance. The paperclippings, or broke, formed in connection with breaks or shutdowns ofthe machine falls into the pocket P formed at the drying cylinder by thedryer wire, from which it may be difficult to remove the broke beforestarting up the machine again.

FIG. 3b shows a slightly different air impingement solution, in whichthe air impingement hood H is fitted over the drying cylinder 20 of adryer section provided with single-wire-wire web transfer, to blow hotair through the wire F, towards the web W. In case of a web break, thebroke can fall down freely under the machine. The maintenance of thehood H may, however, be difficult to arrange. Air impingement arrangedin connection with a conventional drying cylinder takes place throughthe wire, towards the side of the web which is away from the dryingcylinder. In this way, therefore, in addition to the general improvementof drying efficiency, one-sided drying of the web can also be madeuniform.

FIG. 3c shows a part of a dryer section provided with twin-wire webtransfer, in which there are two rows of drying cylinders 20′ and 20″.The air impingement hood H is fitted to blow hot air through the wire Ftowards the web W, as shown in FIG. 3b. In case of a web break, thebroke falls down into a pocket P formed by the lower wire F′. In thecase of FIG. 3d, deviating from the case of FIG. 3c, a lower airimpingement hood H′ is fitted in conjunction with the lower dryingcylinder 20″ in addition to the upper air impingement hood H. In case ofa web break, in this case also, the broke collects in the pocket Pformed by the lower wire F″.

In the case of FIGS. 3b-3 d, hot air is blown through the wire F, F′towards that side of the web W which is away from the drying cylinder20, 20′, 20″, which reduces the one-sidedness of drying and curling ofthe paper. The wire F, F′, however, both disturbs air blowing andreduces the drying efficiency of hot air blowing. As regards energyconsumption, it should be noted that blowing through the wire towardsthe web increases energy consumption in comparison to blowing directlyonto the web. Moreover, a wire made of synthetic material limits thetemperature of the hot air. In air impingement, it is then usually notpossible to blow air hotter than about 300° C. Furthermore, in the casesshown in FIGS. 3b-3 d, the air impingement hoods are fitted inside thewire loops, which hinders access to the hoods and thus also theirmaintenance.

The above-mentioned disadvantages do not occur in the air impingementmodules M₁-M₃ shown in FIGS. 1a and 1 b.

FIG. 4 shows an air impingement unit 428 relating to a specialembodiment of the invention, fitted between the drying cylinder groups430 and 432 provided with single-wire-wire web transfer. The airimpingement unit comprises a linear air impingement hood 434, whichtogether with two linear vacuum boxes 436 and 436′ fitted below the hoodforms a linear slot-like space 438. The paper web 418 is arranged topass through the slot-like space while supported on the dryer wire 440.

In this solution, the paper web is preferably fitted to pass under theair impingement unit, supported by the wire and the underpressurecreated by the vacuum box. The vacuum holds the web in contact with thewire. Without the support of the vacuum, hot air blowing might detachthe web from the wire. To create a vacuum, it is preferable to use avacuum box in this solution, which by means of blasts creates a vacuumthat holds the web to the wire. If so desired, some other type ofsuction box can also be used to create a vacuum.

The wire 440 supporting the paper web passes, supported by rolls 442,for example suction rolls, above the vacuum boxes without touching them,and forms a horizontal track 444 for the web. The wire and the web thusrun, supported by the rolls 442 and the vacuum boxes 436, 436′, at asuitable distance from the cover of the vacuum box and the side 446 ofthe air impingement hood adjacent to the slot. The distance of thenozzle surface 446 of the air impingement unit adjacent to the slot fromthe paper web is typically about 10-50 mm, preferably 15-25 mm.

In the case shown in FIG. 4, below the dryer wire conveying the web 418,two vacuum boxes 436 complying with FIG. 4a are fitted inside the dryerwire loop. The vacuum boxes remove air from the space between the vacuumbox 436 and the wire 440 by means of the blasts depicted by the arrowsa, in the direction of arrow b, thus forming a vacuum in this space. Avacuum of typically about 100-400 Pa, preferably 200-300 Pa is formedbetween the wire and the cover of the box. This vacuum suffices to keepthe web 418 on the wire 440 in a stable manner. The same blasts—arrowsa—keep the wire at a certain distance from the boxes 436, thuspreventing the wire from touching the surface structure 445 of theboxes. Suction rolls 442 or the like guide the passage of the wire 440past the air impingement unit. Below the vacuum boxes, the passage ofthe wire loop is guided by means of conventional turning rolls 450.

The air impingement hood 434 is comprised of a casing-like structure, onwhose mainly planar nozzle surface 446, which is towards the web, thatis, on the side adjacent to the paper web, there are formed a largenumber of nozzles, such as apertures or slots, from which hot air orsteam blasts are blown towards the web. A part of the nozzle surface ofan air impingement hood relating to the invention is shown in FIGS. 5aand 5 b. Several nozzles—in FIGS. 5a and 5 b apertures 535—arepreferably fitted on the nozzle surface 546 in succession, in severaladjacent rows extending across the web. The open area formed byapertures 535 or slots on the nozzle surface 546 is preferably 0.5-5%,more preferably 1-2.5%, the distance between the apertures being 10-100mm (15-35 mm).

As shown by the arrows α in FIG. 5b, the nozzles 535 blow hot air orsteam, preferably approximately at right angles to the web 518, ontothat side of the web which was not in contact with the hot surface ofthe drying cylinders in the previous dryer section 430 (FIG. 4). Thusthe air impingement unit forms a part that reduces curling in the dryersection.

In the solution shown in FIG. 4, the air impingement hood's own returnair is used in the hot air jets of the hood. The air returning from theweb is taken from the hood 434, through the pipes 537 shown in FIGS. 5aand 5 b, into the hood's collection space 539. From the collection spacethe return air is conducted by means of the blower 454 shown in FIG. 4,through a connecting pipe 452 to the heater 456, and from there backagain to be heated and blown by the air hood 434 onto the web. Some ofthe humid return air is removed by means of a fan 458, through a heatexchanger 460, for keeping the air humidity suitable in the return air.New, dry blast air can be fed by means of the blowers 462, 464 throughthe heat exchanger 460 and the burner 456 into the return air.

In the solution shown in FIG. 4, the paper web is passed by a closedtransfer from a drying cylinder group 430 operating by single-wire-wireweb transfer to the wire 440 to be conveyed past the air impingementhood 434. Hot air is blown from the hood towards the web in order toachieve the desired efficient drying impulse, in order to regulatedrying, to eliminate curling and/or to achieve good profiling. Thedrying efficiency of the air blown towards the web can be regulated byadjusting the temperature, humidity or velocity of the hot air jetsblown towards the web. The temperature of the hot air can be regulatedsimply by, for example, adjusting the burner 456. The humidity of thehot air is correspondingly easily adjustable by discharging a greater orlesser part of the humid return air through the blower 458. The velocityof the hot air can be regulated by adjusting the blower 454. In thesolution relating to the invention, the temperature of the hot air jetsdirected at the paper web can be regulated by several degrees in aninstant, which means that the drying efficiency of the hot air jets canalso be adjusted instantly so as to be higher or lower. By means of thesolution relating to the invention it is, therefore, possible toregulate the drying of the paper web to the correct level in a veryshort time, typically in less than 30 seconds, even in less than 10seconds, for example, after a change of grade or a sudden change takingplace in the press section.

In case of a web break, the paper clippings, or broke, in the airimpingement unit is easily removed from the linear section by beingconveyed by the dryer wire, between the air impingement unit and thedrying cylinder group 432 down to floor level, from where it can rapidlybe removed. Also the air impingement unit relating to the invention, notbeing surrounded by a wire loop, in the pocket of which paper clippingsmight collect during a web break, improves the runnability of themachine. The air impingement unit can easily be lifted up and away fromabove the dryer wire 440 for maintenance.

The air impingement unit relating to the invention can be made veryshort—the length of one short air impingement hood and one vacuum box—ifthe aim is, for example, only to eliminate curling of the paper web orto improve profiling. For profiling, even a short intensive dryingimpulse—precision treatment—on the correct part of the web suffices.

On the other hand, the actual drying of the web can also be increased bymeans of the air impingement hood, in which case the air impingementunit can be made longer, if necessary. FIG. 6 shows a longer airimpingement unit comprised of three air impingement hoods 634, 634′,634″. Below each air impingement hood a separate vacuum box 636, 636′,636″ is fitted. The air impingement hoods and vacuum boxes, which are inthemselves linear and straight, are fitted in succession to form acurved slot between them, so that the first air impingement hood 634 andthe vacuum box 636 form a slot 638 directed upwards in relation to theweb, in the direction of travel of the web, the second hood/box pair634′ and 636′ form a horizontal slot 638′, and the third hood/box pair634″ and 636″ form a slot 638″ directed downwards. The slot forms anangle of <45° with the horizontal plane. The angles between the slots638, 638′, 638″ are preferably 5-15°.

FIG. 6 is a drawing of the whole hood 634, as seen obliquely from above.On the hood is marked its division into separate parts or segments 634a, 634 b and 634 c across the web. In the solution relating to theinvention, it is possible to regulate the drying efficiency of thesedifferent segments, which means that, for example, hot air can be blownat lower evaporation efficiency from the outermost segments 634 a and634 c, and air with higher evaporation efficiency from segment 634 b inthe centre. The web often dries more on the edges than in the centre.

FIG. 7 shows an air impingement unit 728 relating to FIG. 4, which,however differs from the above in that the blast air used is not returnair, but instead air taken directly from under the dryer section hood766. The used humid air is not collected from the web area, but insteadthe humid air is allowed to flow freely into the space under the hood.

FIG. 8 shows a typical curve of showing the variation of the dryingefficiency of the drying cylinders in a conventional 68-cylinder dryersection provided with single-wire-wire web transfer. In the first partof the dryer section, at stage 1, is a so-called area of increasingevaporation, in the intermediate part of the dryer section, at stage 2,is an area of constant evaporation, and in the last part of the dryersection, at stage 3, is an area of diminishing evaporation.

At the top of FIG. 8 is shown additionally, by way of an example asregards a dryer section,

the optimum area for air impingement, that is, the areas in the dryersection in which the web can advantageously be influenced by means ofair impingement, in comparison to cylinder drying,

the optimum area for profiling, that is, the area in the dryer sectionin which the profiling of the web, that is, the cross-direction dryingof the web, can best be influenced,

the optimum area for controlling curling, that is, the area in whichcurling due to one-sided drying of the web can best be reduced,

the optimum area for controlling curling, in which curling can best beinfluenced by a drying impulse, e.g. by means of air impingement ofshort duration, and

the optimum area for combined increasing of drying efficiency, profilingand curling control.

The optimum area for performing air impingement in order to affect allthe foregoing regulation of drying has been found to be within the rangewhere the paper web has already dried to a dry-matter content of >70%,but not yet 95%, preferably within the range where the paper web hasalready dried to a dry-matter content of >75%, but not yet 85%. Theoptimum area for air impingement often falls within a range where theweb has a dry-matter content of approximately 75%-80%. In the dryersection provided with single-wire-wire web transfer shown in FIG. 5,which comprises almost 70 drying cylinders, this optimum area fallsapproximately between the 48th and the 61st drying cylinder.

FIG. 9 shows an example of the profiling of a printing paper webaccording to a solution relating to the invention. The upper curve inthe figure shows the moisture profile, without separate profiling, ofthe paper web coming from the dryer section which incorporates a 5 mlong air impingement unit in addition to the drying cylinders. Theaverage moisture content of the paper web is 5.3%, that of the edgeareas 4.1% and that of the central area 6.5%, while the temperature ofthe air impingement air is approximately 300° C. and velocity a constant80 m/s across the entire web.

The profiling of the paper web is done by closing the outermost segmentsof the air impingement unit and by increasing the blowing velocity ofthe middle segment to 150 m/s. In this way, the average moisture levelremains almost the same, that is, at 5.1%, the moisture level of theedges increases and that of the middle part decreases. The moisturevalues vary between 4.7 and 5.4. The web's moisture profile has thusbecome markedly more uniform, as can be seen from the lower curve inFIG. 6.

The primary purpose of the linear air impingement unit relating to thespecial embodiment of the invention is not necessarily to removemoisture from the web in a conventional manner. Since the evaporationefficiency of the air impingement dryer can be changed very rapidly, thedrying efficiency of the dryer section as a whole can be regulated morerapidly than in conventional cylinder drying, although there may only beone dryer relating to the invention in the machine. This may be utilisedadvantageously in a change of grade, where with conventional devicesolutions, there often occur even relatively long periods during whichthe dry matter content “creeps” before reaching a stable status. Bymeans of a rapidly reacting air impingement unit these phenomena can bereduced decisively and even completely eliminated.

The fact that, due to the rapid regulation, the shut-down and start-upof the machine can be accomplished faster may be considered a notableadvantage of the solution relating to the invention. The structure ofthe air impingement unit also contributes to improving the runnabilityof the machine because, for example, the removal of broke in the area ofthe unit is easily performed.

The air impingement unit is suitable for use in controlling the curlingof the web when the drying energy is brought through that surface of theweb which has not been in contact with the preceding drying cylinders.The linear air impingement unit can also be fitted in a manner differingfrom the solutions shown in FIGS. 2-4, so that the slot through whichthe web passes is vertical, if this is desirable for reasons of use ofspace or other reasons.

The air impingement units, which are relatively small in size, caneasily be fitted in an existing dryer section to improve the operationof the dryer section, moisture content control, curling and profiling.The air impingement units can easily be divided into separate segments,which make it possible to use the unit for regulating the moistureprofile in the cross direction of the web.

The invention is described in the foregoing with reference to only someof its advantageous embodiments, to the details of which the inventionis not, however, intended to be strictly limited. Many modifications andvariants are possible within the scope of the inventive idea specifiedin the claims presented below.

What is claimed is:
 1. A method of drying a paper web using a pluralityof heated drying cylinders, and at least one air impingement modulehaving a hood for blowing a blowing medium directly against the paperweb, comprising: (a) bringing the paper web into operative contact withthe heated drying cylinders; and (b) regulating at least one of finalmoisture content, quality, and cross-direction profile of the paper webby adjusting an efficiency of air impingement drying of the paper web byadjusting at least one of a blowing velocity of a medium blown directlyagainst the paper web, a humidity of the blowing medium, a temperatureof the blowing medium, and a distance of the hood of the at least oneair impingement module from the paper web.
 2. A method as recited inclaim 1 wherein b) is practiced to regulate every one of the finalmoisture content, quality, and cross-direction profile of the paper webby adjusting every one of the blowing velocity of the medium blownagainst the paper web, the humidity of the blowing medium, thetemperature of the blowing medium, and the distance of the hood of theat least one air impingement unit from the paper web.
 3. A method asrecited in claim 1 wherein the drying cylinders are in a plurality ofgroups; and further comprising (c) also regulating at least one of finalmoisture content of the paper web and its quality by adjusting a steampressure of one or more drying cylinder groups.
 4. A method as recitedin claim 1 further comprising (c) measuring the final moisture contentof the paper web after substantially a last drying cylinder, and (d)utilizing the measurement from (c) to adjust the efficiency of the airimpingement drying in (b).
 5. A method as recited in claim 3 furthercomprising measuring the final moisture content of the paper web afterthe last drying cylinder group, and adjusting the steam pressure of oneor more drying cylinder groups in response to that measurement tothereby regulate the final moisture content of the paper web.
 6. Amethod as recited in claim 4 wherein (b) is practiced to regulateproperties of the paper web across the entire width thereof.
 7. A methodas recited in claim 3 wherein at least one of (b) and (c) are practicedby regulation on the basis of model predictive multi-variable control.8. A method as recited in claim 3 wherein at least one of (b) and (c)are practiced by regulation based on an optimization algorithm whichoptimizes at least one of quality and production costs of the paper web.9. A method as recited in claim 1 wherein (b) is practiced by at leastone of adjusting machine-direction segments of the air impingement hoodseparately, and by closing machine-direction segments of the airimpingement hood.
 10. A method as recited in claim 1 further comprising(c) effecting a change of paper web grade, and (d) during the change ofpaper web grade, regulating the drying efficiency by at least one of (i)using information on the paper web grade or models describing the changeof paper web grade to control the steam pressures of the dryingcylinders, (ii) by controlling one or more air impingement modules ofthe at least one air impingement module on the basis of a target value,and (iii) by using continuous feedback control from a measuring devicewhich measures final moisture content downstream of a last dryingcylinder.
 11. A method as recited in claim 1 further comprising: (c)during start-up of the method, preheating the drying cylinders and theair impingement hoods of the at least one air impingement module;setting running parameters of the web by controlling air impingementfrom the module and by adjusting a steam pressure of the dryingcylinders on the basis of feedback from a measurement of final moisturecontent of the web downstream of a last drying cylinder; and regulatingair impingement as a drying efficiency of the drying cylinders changes.12. A method as recited in clam 1 further comprising: (c) during a breakof the paper web switching steam pressures of the drying cylinders andefficiencies of the at least one air impingement module to break values;and (d) after the break is repaired or remedied, switching the steampressures of the drying cylinders and the efficiency of the least oneair impingement module back to normal running values, and correcting thepaper web quality by adjusting air impingement parameters.
 13. A methodof treating a paper web in a paper machine having a press section and adrying section, the drying section including at least one dryingcylinder group, and a wire passing through a slot-like space formedbetween the curved or linear air impingement hood and a curved or linearsurface, said method comprising: (a) passing the paper web from thepress section through at least one drying cylinder group; (b) drying thepaper web in the at least one drying cylinder group to an average drymatter content of greater than 70%; and (c) guiding the paper web fromthe at least one drying cylinder group supported on a wire through theslot-like space formed between the air impingement′hood and surface, andblowing several successive air or steam jets directly against the web inthe slot-like space in the cross-web direction in order to regulate thedry matter content of the paper web.
 14. A method as recited in claim 13further comprising (d) after the dryer section or drying cylinder group,measuring the dry matter content of the paper web; and wherein (c) ispracticed to regulate the dry matter content of the paper web byadjusting the drying efficiency of the air or steam jets according tothe measurements from (d).
 15. A method as recited in claim 14 whereinat least two successive air impingement hoods are provided extendingacross the web; and wherein (c) is practiced by separately adjusting thedrying efficiency of the air or steam jets coming from each airimpingement hood.
 16. A method as recited in claim 13 using at least twoair or steam jet groups in a cross-web direction; and wherein (c) ispracticed to regulate the dry matter content of the paper web in thecross-web direction by at least one of separately adjusting the dryingefficiencies of the at least two air or steam jet groups in successionin the cross-section direction, and separately adjusting the dryingefficiency of each air jet in succession in the cross-web direction. 17.A method as recited in claim 13 wherein (c) is practiced by blowing airjets toward the paper web, and regulating the drying efficiency of theair jets by adjusting the temperature of the air jets between about200-400° C.
 18. A method as recited in claim 13 wherein (c) is practicedby blowing air jets toward the paper web, and by regulating the drymatter content of the paper web by adjusting the drying efficiency ofthe air jets by adjusting the moisture content so that it varies betweenabout 0-300 gH₂O/kg of dry air.
 19. A method as recited in claim 13wherein (c) is practiced by blowing air jets toward the paper web, andby regulating the dry matter content of the paper web by adjusting thedrying efficiency of the air jets by adjusting the velocity of the airjets within the range of about 50-150 meters per second.
 20. A method asrecited in claim 13 wherein (c) is practiced by blowing air jets towardthe paper web and using as the source of air at least one of replacementair from the paper machine room, return air of a hood and a dryersection closed by a hood, and return air from the impingement drying;and further comprising at least one of increasing the temperature, anddecreasing the moisture level, of the air before blowing it toward thepaper web.
 21. A method as recited in claim 13 wherein two or more airimpingement hoods or air impingement hood sections are utilized; andfurther comprising: (d) blowing return air from the two or more airimpingement hoods or air impingement hood segments, toward the paperweb; and further comprising heating the return air before it is directedto the air impingement hoods or segments in at least one of a commonarea for all of the air impingement hoods or segments, and a separatearea for each air impingement hood or segment.
 22. A method as recitedin claim 13 wherein (c) is practiced so that the average dry mattercontent of the paper is increased to within the range of 75-95%.
 23. Amethod as recited in claim 13 wherein (c) is practiced utilizing two ormore successive air impingement hoods, and wherein (c) is practiced sothat the drying efficiency of the air jets from the first airimpingement hood is on average higher than the drying efficiency of airjets from the following air impingement hoods.
 24. A dryer section of apaper machine for drying a paper web, comprising: a plurality of dryingcylinders for operatively engaging the paper web; at least one airimpingement module, having an air impingement hood, for blowing a fluidmedium directly against the paper web; a measuring device which measuresthe final moisture content of the paper web passing through the dryersection; and means for adjusting at least one of the blowing velocity,temperature, and humidity of the fluid medium, and the distance of theair hood from the paper web.
 25. A dryer section as recited in claim 24further comprising means for regulating the steam pressure of saiddrying cylinders.
 26. A dryer section as recited in claim 24 furthercomprising at least one control unit for regulating the efficiency ofsaid at least one air impingement module in response to the measurementsfrom said measuring device.
 27. A dryer section as recited in claim 26wherein said plurality of drying cylinders are in groups including afirst group which operatively engages a highest moisture content portionpaper web, and a last group which operatively engages a lowest moisturecontent portion of the paper web; and wherein said measuring device islocated downstream of said last drying cylinder group.
 28. A dryersection as recited in claim 27 further comprising means for regulatingthe steam pressure of the drying cylinders of one or more of said dryingcylinder groups.
 29. A dryer section as recited in claim 26 furthercomprising a memory operatively connected to said at least one controlunit for regulating drying efficiency and supplying paper grade-specificinformation to said control unit.
 30. A dryer section as recited inclaim 24 further comprising means for measuring at least one of anyquality of the paper web, and the cross-direction profile of the paperweb; and means for measuring the blowing efficiency of at least one ofsaid at least one air impingement module, and the cross-directionprofile of said at least one air impingement module.
 31. Apparatus foroptimizing at least one of the drying of a paper web in a paper machine,and the power consumption in the dryer section of the paper machine,comprising: at least one first drying cylinder group which dries a paperweb to an average dry matter content of greater than 70%; at least oneair impingement module which further controls drying of the paper webafter exiting said at least one first drying cylinder group; and said atleast one air impingement module comprising: an air impingement hoodextending across at least the paper web; a curved or linearcounter-surface extending across at least the paper web, and definingwith said hood a slot-like space therebetween through which the paperweb can pass; and means for blowing several air or steam jets directlyagainst the paper web as it passes through said slot-like space. 32.Apparatus as recited in claim 31 wherein said curved or linearcountersurface comprises a suction roll or a vacuum box.
 33. Apparatusas recited in claim 31 further comprising a last drying cylinder group;and wherein said at least one air impingement module is located betweensaid at least one first drying cylinder group and said last dryingcylinder group.
 34. Apparatus as recited in claim 31 further comprisinga last drying cylinder group; and wherein said at least one airimpingement module is located substantially immediately after said lastdrying cylinder group.
 35. Apparatus as recited in claim 31 wherein saidat least one air impingement module comprises two or three successiveair impingement hoods in the direction of paper travel, and two or threesuction rolls or vacuum boxes defining said curved or linearcounter-surface in the direction of travel of said paper web; andwherein said hoods and counter-surfaces define a substantiallyhorizontal slot-like space between them.
 36. Apparatus as recited inclaim 31 wherein at least one air impingement module comprises an airimpingement hood cooperating with two or three vacuum boxes insuccession in the direction of travel of the paper web, said vacuumboxes positioned below said air impingement hood to form a substantiallyhorizontal slot-like space therebetween.
 37. Apparatus as recited inclaim 31 further comprising a wire extending through said slot-likespace to facilitate transport of a paper web therethrough; and furthercomprising means for creating a vacuum beneath said wire.
 38. Apparatusas recited in claim 31 further comprising an endless wire loop forsupporting the paper web in said slot-like space; and wherein saidcounter-surface is mounted within said endless wire loop.
 39. Apparatusas recited in claim 31 further comprising an endless wire loop fortransporting the paper web through said slot-like space; and a pluralityof turning rolls for supporting the paper web in said slot-like space.40. Apparatus as recited in claim 31 wherein said at least one airimpingement module comprises a linear dryer section; and furthercomprising at least one drying cylinder group following said lineardryer section; and a closed paper web transfer from said first dryingcylinder group to said linear dryer section, and from said linear dryersection to said last drying cylinder group.
 41. Apparatus as recited inclaim 31 wherein said at least one impingement module comprises two ormore air impingement hoods, or air impingement hood sections; andfurther comprising a common means for at least one of heating return airdirected toward said two or more air impingement hoods or segments, andreducing the humidity of return air directed to said two or more airimpingement hoods or segments.
 42. Apparatus as recited in claim 31wherein said at least one impingement module comprises two or more airimpingement hoods or air impingement hood segments; and furthercomprising integrated means within each of said air impingement hoods orsegments for at least one of heating return air and reducing thehumidity of return air received by said air impingement hoods orsections.
 43. Apparatus as recited in claim 31 wherein said airimpingement hood comprises two or more rows of aperture or slit nozzlesextending across the paper web for blowing air or stream jets toward thepaper web.
 44. Apparatus as recited in claim 43 wherein said aperture orslit nozzles are formed in a substantially planar perforated or slottedplate forming the bottom of the air impingement hood and substantiallyparallel to the paper web; and wherein said perforated or slotted plateis adjacent to said slot-like space, and wherein the open area of saidperforated or slotted plate is between about 1-2.5%, and the distancebetween apertures is between about 15-35 mm.
 45. Apparatus as recited inclaim 31 wherein said at least one air impingement module comprises alinear dryer section having a length of about 5-10 meters.
 46. Apparatusas recited in claim 31 wherein said at least one impingement modulecomprises a linear dryer section; and wherein said linear dryer sectionis positioned in an inclined or vertical position so that the paper webwill pass through said slot-like space in a plane deviating from thehorizontal.